By three years old, human children start to display empathy and will intervene in conflicts where they feel someone has been wronged. Chimpanzees, on the other hand, seem to be stuck in the terrible twos — like children in that notoriously selfish stage, they will only intercede when they have been directly harmed.

Researchers call the act of sticking up for others “third-party punishment.” We do it, chimps don’t, and this distinction between humans and our closest evolutionary relatives may be an important clue in understanding the development of wide-scale cooperation in modern humans, according to Keith Jensen, a psychologist at Queen Mary, University of London.

He is a co-author of a study, published in September in the Proceedings of the National Academy of Sciences, in which researchers determined that while chimpanzees regularly punish individuals who steal from them personally, they do not intervene after observing a similar transgression against another individual. The results provide “the closest glimpse we have of our last common ancestor,” Jensen says.

While human societies around the world have some variation of a police force and courts of law, chimpanzees don’t appear to share our instinct for arbitration. “One of the big puzzles about humans is that we cooperate in such large groups,” says Robert Boyd, an anthropologist at the University of California in Los Angeles, who was not involved in the study.

For non-humans, third-party punishing behavior poses a potentially high risk to an individual’s social standing — intervening in someone else’s dispute could jeopardize one’s own position if the mediation is ignored. But as humans evolved and began to signal to one another that multiple members of the group were willing to punish a violation, the cost of punishing shifted from one to many and decreased accordingly, making it a more viable option, Boyd said. Once third-party punishment became the norm, it encouraged cooperation by preventing free-riders from taking advantage of the work of others.

The new chimpanzee study contradicts past research in which scientists saw what they believed to be third-party punishment in chimpanzees and other animal species. However, those prior studies relied almost exclusively on observations of the primates’ unprompted behavior. The previous studies have the same potential pitfalls as watching people on the street. “If you see someone scold a jay-walker, there could be so many different motivations for that,” Jensen says. “With chimpanzees this is especially important because you can’t ask them.”

In Jensen’s experiment, one chimpanzee could steal another’s food by pulling it along a rope, sliding it towards him. A third chimp in the observer position then had the option to release a trap door, dropping the food out of reach and denying the thief the fruits of his labor.

To determine if the chimps understood the arrangement, the scientists also gave them the opportunity to release the trap door when they were directly stolen from. Victims regularly defended themselves when their food was stolen, and dominant individuals were the most likely to seek retribution. In contrast, neither dominant nor subordinate individuals consistently released the door when their compatriot was the victim.

Not everyone is convinced that the experiment captured the whole range of chimpanzee punishment, however. A stolen meal may simply be too minor a transgression to trigger third-party intervention, according to Carolus van Schaik, who heads the Anthropological Institute and Museum at the University of Zurich. Interventions may be much more likely, he wrote in an email, when the social stability of a chimp group is threatened, such as when new members are introduced or after the dominant individual has died, leaving an open position.

Though researchers still argue over whether there are certain situations in which chimps may engage in third-party punishment, no one thinks it happens often.

Jensen’s work ultimately suggests that “chimpanzees are much more egocentric and self-interested than humans, and less interested in the welfare of others,” Boyd says.

So the next time your child is behaving like an ape, be thankful that, like the human species, she’ll outgrow it.